The present invention relates to a radio frequency or optical communication system in which a relay station is used to aid communication among a network of parties, and more particularly to an improvement allowing more efficient use of the available channel resource.
Radio frequency or optical communication among a network of physically separate devices using a third party relay station is illustrated in FIG. 1. The relay station 10 receives a signal S.sub.1, S.sub.2, S.sub.3, . . . , and/or S.sub.N from each of the user devices 12, 14, 16, . . . , 18 and retransmits the composite received signal S.sub.1 +S.sub.2 +S.sub.3 + . . . +S.sub.N back to all of the user devices 12, 14, 16, . . . , 18, typically on a frequency channel offset from the receiving frequency channel. Two examples are:
1. Satellite communication among a network of earth terminals (ground, airborne, or shipboard) using a non-processing "bent pipe" transponder. PA1 2. Terrestrial line-of-sight radio communication among a network of terminals, using a relay radio to connect them (such as in a tethered aircraft or a radio tower). PA1 The round-trip propagation delay to and from the relay station; and PA1 Relative signal amplitude, carrier frequency and phase (if applicable to the selected modulation waveform) of the relayed source signal vs. the a priori known source signal.
The relay resource is typically shared using any of the well-known multiple access techniques such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) or hybrids which use one or more of these techniques in combination. In each of these techniques, the relay resource is effectively channelized, so that the receiver can, by tuning in on a specific frequency, at a specific time, with a specific code, or some combination thereof, isolate a particular received signal from all the others. For simplicity, the following discussion will be based on the FDMA technique, although the apparatus may be used with TDMA, CDMA, or hybrid transmission and reception techniques.
Often one (or more) of the user devices or network stations is more capable than the others, being able both to transmit wider bandwidth signals at a higher power and to receive and process lower power signals from multiple sources simultaneously. In such networks, the more capable station 12 is often termed the "hub" while the less capable stations 14, 16, . . . , 18 are termed "spokes". The communication channels in such a network consist of one wide bandwidth channel for the transmissions from the hub 12 to all the spokes 14, 16, . . . , 18 and multiple smaller bandwidth channels for the transmissions from each of the spokes 14, 16, 18, . . . , 18 to the hub 12.
FIG. 2 shows a set of N signals associated with a hub and spoke network separated in the frequency domain. In this example, User 1 is the hub 12, and Users 2 to N are the spokes 14, 16, . . . , 18. The signal S.sub.1 transmitted by User 1 has a center frequency of f.sub.1 Hz and a bandwidth of W.sub.1 Hz; the signal S.sub.2 transmitted by User 2 has a center frequency of f.sub.2 Hz and a bandwidth of W.sub.2 Hz; and so on for all N users in the network. With the FDMA approach, the total bandwidth required to support simultaneous transmissions from all of users in the network is the sum of the individual bandwidths (W.sub.1 +W.sub.2 + . . . +W.sub.N) Hz, as all of the signals being relayed are assigned to non-overlapping bandwidths. The bandwidth of the relay therefore imposes a limit on the number of users. Although this is a straightforward partitioning of the relay resource among a number of users, it is not necessarily the most efficient use.
What is needed is a mechanism to provide more efficient use of the available channel resource to support relayed communication among a network of radio frequency or optical terminals of differing capabilities. More specifically, what is needed is an improvement which is independent of the multiple access technique in use, independent of the modulation and coding of the system and which is independent of any data compression techniques or need for special equipment at the relay station.